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Commit 5b04ec4a authored by Baurzhan Ismagulov's avatar Baurzhan Ismagulov Committed by Greg Kroah-Hartman
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serial: cpm_uat: reducing CPM serial latency 



Setting Rx FIFO size to 1 reduces reader process wakeup latency up to
20x for baud rates <= 9600. This patch sets the Rx FIFO size to 1 for
baud rates <= 9600 or if low latency has been requested for the tty.

Signed-off-by: default avatarMichael Trimarchi <michael@gandalf.sssup.it>
Signed-off-by: default avatarBruno Morelli <bruno@evidence.eu.com>
Signed-off-by: default avatarBaurzhan Ismagulov <ibr@radix50.net>
Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@suse.de>
parent 63295cb2

drivers/serial/cpm_uart/cpm_uart_core.c

+19 −0
Original line number Diff line number Diff line
@@ -72,6 +72,8 @@ static void cpm_uart_initbd(struct uart_cpm_port *pinfo); 


/**************************************************************/



#define HW_BUF_SPD_THRESHOLD    9600



/*

 * Check, if transmit buffers are processed

*/
@@ -503,6 +505,11 @@ static void cpm_uart_set_termios(struct uart_port *port, 
	pr_debug("CPM uart[%d]:set_termios\n", port->line);



	baud = uart_get_baud_rate(port, termios, old, 0, port->uartclk / 16);

	if (baud <= HW_BUF_SPD_THRESHOLD ||

	    (pinfo->port.state && pinfo->port.state->port.tty->low_latency))

		pinfo->rx_fifosize = 1;

	else

		pinfo->rx_fifosize = RX_BUF_SIZE;



	/* Character length programmed into the mode register is the

	 * sum of: 1 start bit, number of data bits, 0 or 1 parity bit,
@@ -594,6 +601,17 @@ static void cpm_uart_set_termios(struct uart_port *port, 
	 */

	bits++;

	if (IS_SMC(pinfo)) {

		/*

		 * MRBLR can be changed while an SMC/SCC is operating only

		 * if it is done in a single bus cycle with one 16-bit move

		 * (not two 8-bit bus cycles back-to-back). This occurs when

		 * the cp shifts control to the next RxBD, so the change does

		 * not take effect immediately. To guarantee the exact RxBD

		 * on which the change occurs, change MRBLR only while the

		 * SMC/SCC receiver is disabled.

		 */

		out_be16(&pinfo->smcup->smc_mrblr, pinfo->rx_fifosize);



		/* Set the mode register.  We want to keep a copy of the

		 * enables, because we want to put them back if they were

		 * present.
@@ -604,6 +622,7 @@ static void cpm_uart_set_termios(struct uart_port *port, 
		out_be16(&smcp->smc_smcmr, smcr_mk_clen(bits) | cval |

		    SMCMR_SM_UART | prev_mode);

	} else {

		out_be16(&pinfo->sccup->scc_genscc.scc_mrblr, pinfo->rx_fifosize);

		out_be16(&sccp->scc_psmr, (sbits << 12) | scval);

	}